1. Field of the Invention
The present invention relates to computed tomographic (CT) imaging, and in particular to CT imaging with interlacing based data upsampling.
2. Discussion of the Background
Images produced from helical multislice CT systems exhibit a distinct type of artifact when operated at medium and high helical pitch values. The artifacts appear as alternating light and dark regions around structures whose features change axially. The shape of the artifact is similar to the vanes on a windmill, hence the name “windmill” artifact (this artifact has also been referred to as simply the “helical” artifact in the literature.)
The cause of the windmill artifact is insufficient sampling by the detectors in the axial direction. The windmill artifact can be decreased or eliminated if axial sampling (resolution) can be increased by decreasing the size of the detector in the axial direction. However, this is not a practical alternative since detectors are already at the smallest limit supported by the current level of technology.
One method of increasing the axial sampling without physically reducing the size of the detectors is to implement an x-ray tube with a flying focal spot is described in U.S. Pat. No. 6,256,369, and is shown in FIG. 1. REB represents the position of the electron beam on the anode in the radial direction, which, due to the angled shape of the anode, has a corresponding focal spot position zfs in the axial direction. The nominal source-object and source detector distances are SOD and SDD, with nominal detector size at isocenter w. Deflection of the electron beam position REB to ±ΔSOD results in two focal spots at zfs=±δ in the axial direction (with actual source-object and source-detector distances SOD±ΔSOD and SDD±ΔSOD). When δ is calculated to produce an offset of one quarter of the detector widths at isocenter (see Stierstorfer Flohr, “A Reconstruction Procedure for CT Systems with z-Sharp Technology,” Proc. 8th Annual Meeting on Fully Three Dimensional Image Reconstruction in Radiology and Nuclear Medicine, July 2005, pp. 28-30).
                    δ        =                              1            4                    ⁢                                    w              ·              SDD                                      (                              SDD                -                SOD                            )                                                          (        1        )            the detector segments of adjacent views will overlap by half their width:
                              w          FFS                =                  w          2                                    (        2        )            Simply backprojecting the flying focal spot data as-is will not result in a doubling of axial sampling density with little or no windmill artifact reduction, since the width of the detectors is still w.
Siemens incorporates a flying focal spot in their Sensation 64 CT scanner for the same application. In their implementation, while exact, the projection data is rebinned and resampled in both the longitudinal and azimuthal direction into a parallel geometry, and then reconstructed (see Kachelreiβ, Knaup, Penβell, and Kalender; “Flying Focal Spot in Cone-Beam CT,” IEEE Transactions on Nuclear Science, Vol. 53, No. 3, June, 2006, pp. 1238-1247, and Stierstorfer et al., supra. Philips also has described a rebinning and resampling to parallel reconstruction algorithm for flying focal spot in Shechter, Koehler, Altman, and Proksa, “High-Resolution Images of Cone-Beam Collimated Scans,” IEEE Transactions on Nuclear Science, Vol. 52, No. 1, February, 2005, pp. 247-255. Rebinning and resampling is a time-consuming task that does not lend itself easily to cone-beam geometry. It also requires approximately twice the number of views to obtain the same signal-to-noise ratio as the non-flying focal spot case.
A common zero-interlacing applications is increasing the data rate of a digitized signal (see Elliot, Douglas, ed., Handbook of Digital Signal Processing: Engineering Applications, Academic Press, Inc., 1987, pp. 234-237; also referred to as “zero packing”). If a digitized signal is sampled with some period T, the data can be upsampled by a factor of N to T/N by packing the upsampled data values with zeroes, taking the FFT, low pass filtering, and then taking the inverse FFT. This resulting upsampled values are interpolated values, however, and do not represent a true increase in the sampling density. If implemented without flying focal spot, it would have little or no effect on the windmill artifact.
A “zero-interleaving” technique applied to CT was described in Shechter, Koehler, Altman, and Proksa, “High-Resolution Images of Cone-Beam Collimated Scans,” IEEE Transactions on Nuclear Science, Vol. 52, No. 1, February, 2005, pp. 247-25. In that technique, zero-interleaving of data in the channel direction was used to implement a faster version of the ray offset technique improving transaxial resolution. This is not a flying focal spot algorithm and has no increase in sampling in the axial direction. Additionally, like the above Siemens approach, it requires rebinning and resampling into a parallel geometry.